The present invention relates to a rheological measurement method and apparatus. More particular, the present invention relates to a method and apparatus for the in-line measuring of the characteristics of a molten material such as viscosity, melt volume rate and melt flow index during an extrusion process.
Rheological testing equipment and methods have become invaluable in recent years in improving the process control of the manufacture of plastic products from polymer melts. Such equipment allows for the determination of rheological properties of the melts such as melt flow index (MFI), melt volume rate (MVR) and viscosity. This, in turn, has allowed manufacturers to exert control over the plastics manufacturing process and produce consistent, high quality products by allowing them to make adjustments to the manufacturing process parameters.
Early rheological testing equipment was primarily used in conducting laboratory measurements of these characteristics of polymer melts. Based on the results of these tests, adjustments were then made in the manufacturing process in an attempt to fine-tune the process. While helpful, such tests are limited in that they cannot provide xe2x80x9creal-timexe2x80x9d results during the manufacturing process, allowing for near instant and constant adjustments to be made to the process in order to optimize the quality and consistency of the resulting products.
In response to this shortcoming, more recent efforts have been directed toward measurement of these properties on-line or in-line during the manufacturing process, thereby providing quicker and continuous feedback to a process controller and allowing closer control over the quality of the polymer melt. One such type of on-line system includes a rheometer that diverts a portion of the plastic melt from the main stream of molten plastic, conducts rheological measurements on the diverted melt, and then purges the melt to the atmosphere or external repository. Such on-line rheometers are contrasted with in-line rheometers, which return the diverted melt portion to the main melt stream after testing. In-line rheometers eliminate the waste generated by the discharge of the plastic melt after measuring in on-line systems.
In both types of rheometers, a metering pump is typically used to feed a capillary passage with a controlled amount of the diverted melt. The pressure drop along the capillary passage is measured and the temperature of the diverted melt is closely controlled with an independent heating or cooling system in order to determine MFI and/or MVR. More recently, on-line rheometers without a metering pump have been developed in which the diverted melt is pushed through a temperature controlled capillary passage under the pressure difference between the main stream and atmospheric pressure. For this type of on-line rheometer without a metering pump, MFI and MVR can be determined by measuring pressure drop and discharge rate along the capillary passage.
Nevertheless, a need exists for an in-line rheological measuring system that is simple, easy to install, operate and maintain, and can instantly and easily measure MFI and MVR on the entire melt stream in real time during a plastic extrusion process to provide quick and accurate quality control data and feedback information to process control. In addition, it would be helpful to be able to determine process control information as temperature, shear stress and shear rate continuously vary, overcoming the deficiencies of prior extrusion rheometer systems that required that temperature and shear rate be fixed. The in-line measured MFI and MVR should match quality assurance laboratory results, with the difference between the in-line and laboratory measurements falling below the standard deviation of the laboratory measurement itself.
A system for conducting in-line rheological measurements on a process melt for providing process control information includes a vessel or die containing a polymer melt under pressure, the vessel or die having a head portion through which the polymer melt may flow, a feeding system for providing a polymer to the vessel or die, a polymer melt temperature sensor, a processing rate sensor, a vessel or die pressure sensor, a vessel head or die head temperature sensor, and a controller responsive to signals provided by the sensors for determining process characteristics of the polymer melt.
A system for providing process control information concerning a polymer melt in an extrusion process comprises a processing rate sensor for monitoring a rate at which the polymer resin is added to the extrusion process, a polymer melt temperature sensor for monitoring the temperature of the polymer melt prior to its extrusion, an extrusion die pressure sensor for monitoring the pressure inside the extruder, a die head temperature sensor for monitoring the temperature of the die head, and a controller responsive to signals generated by said sensors.
A method for providing process control information regarding a polymer melt in an extrusion process includes the steps of adding a polymer resin to an extruder, heating the polymer resin to form a molten polymer melt, elevating an internal pressure of the extruder to a pressure greater than atmospheric pressure, discharging the polymer melt through one or more die opening of any constant geometry defined in a die head coupled to the extruder, measuring the rate at which the polymer resin is added to the extruder, measuring the temperature of the polymer melt just prior to its being discharged, measuring the temperature of the die head, measuring the internal die pressure of the extruder, and conveying the measurements to a controller.